The present invention relates generally to a method for determining joint stiffness degradation and more particularly to a non-destructive method for determining joint stiffness degradation.
NVH (noise, vibration, and harshness) performance can be a major concern in many product designs. Many products, such as automobiles, may experience NVH degradation over the life of the product. This degradation may result from a combination of age and use (mileage in the case of automobiles). NVH degradation can result in a reduction of product performance and customer satisfaction. This is highly undesirable.
In the case of automotive designs, one cause of high mileage NVH degradation is a loss of joint stiffness. The loss of joint stiffness from the body structure can result from fatigue, loosening, aging, wear, corrosion, and a host of other causes. In order to design vehicles with reduced NVH degradation, it often is important to analyze designs to determine locations of body joint stiffness degradation due to high mileage. By isolating such positions, joint stiffness may be modified and improved to avoid future NVH degradation.
Conventional test methods for determining joint stiffness typically require cutting the joints off a vehicle body. Stiffness degradation of each joint is determined by the difference of stiffness before and after mileage accumulation. Since the test requires destroying the vehicle body by cutting off the joints, at least two vehicles are required (one for initial testing and a second after mileage accumulation) adding to the time and cost of the test. In addition, the process of cutting the joints off the vehicle body and performing conventional joint stiffness tests can also be an expensive and time-consuming procedure. Finally, since the conventional procedure requires separate vehicles to be cut and tested for a low versus high mileage stiffness testing, the potential for significant variabilities in both the cut joints and testing procedures are possible. This may result in unreliable stiffness degradation predictions. A test procedure that could be performed non-destructively on a single vehicle body before and after mileage accumulation could reduce expenses, reduce time for testing, and increase testing accuracy. While it is possible to use standard body modal and static bending/torsion stiffness tests to determine the reduction in overall body stiffness and natural frequency due to high mileage, these tests do not identify local joint stiffness degradation which can be crucial in the prediction and prevention of high mileage NVH degradation.
It would, therefore, be highly desirable to have a non-destructive method for determining joint stiffness degradation. It would further be highly desirable to have a method for determining joint stiffness degradation that reduce the expense and time often associated with such procedures. Finally, it would be desirable to have a method for determining joint stiffness degradation that could potentially reduce testing variabilities.
It is, therefore, an object of the present invention to provide a method for determining joint stiffness degradation that is non-destructive, can reduce the time and cost of testing, and reduce hardware test variabilities.
In accordance with the objects of the present invention, a method for determining joint stiffness degradation is provided. The method includes a first exciting of the structure and a first measuring of transfer function and frequency response function. The method also includes simulating a mileage accumulation process of the structure. After the mileage accumulation process of the structure, a second exciting of the structure and a second measuring of frequency response function is undertaken. The first measuring and the second measuring are then used to calculate change in joint stiffness.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and the appended claims.